a017: 五則運算 FYI: https://yunlinsong.blogspot.com/2016/02/blog-post_4.html

a017.cpp

#include <iostream>
#include <string>

using namespace std;

void eatspaces(string &str);          // Function to eliminate blanks
int expr(string &str);             // Function evaluating an expression
int term(string &str, int& index);   // Function analyzing a term
int number(string &str, int& index); // Function to recognize a number
string extract(string &str, int& index); //Function to extract a substring

int main(void)
{
   string buffer;    // Input area for expression to be evaluated

   while( getline(cin, buffer) )
   {
      eatspaces(buffer);                    // Remove blanks from input

      cout << expr(buffer)      // Output value of expression
           << endl;

      //cout << buffer << endl;
   }
}


// Function to eliminate blanks from a string
void eatspaces(string &str)
{
    for(unsigned i = 0; i < str.length(); i++)
    {
        if(str[i] == ' ')
            str.erase(i, 1);
    }
}


// Function to evaluate an arithmetic expression
int expr(string &str)
{
   int value = 0;          // Store result here
   int index = 0;             // Keeps track of current character position

   value = term(str, index);  // Get first term

   for(;;)                    // Infinite loop, all exits inside
   {
      switch(str[index++])  // Choose action based on current character
      {
         case '\0':                 // We're at the end of the string
            return value;           // so return what we have got

         case '+':                         // + found so add in the
            value += term(str, index);     // next term
            break;

         case '-':                         // - found so subtract
            value -= term(str, index);     // the next term
            break;

         default:                       // If we reach here the string
            cout << endl                // is junk
                 << "Arrrgh!*#!! There's an error"
                 << endl;
            exit(1);
      }
   }
}


// Function to get the value of a term
int term(string &str, int& index)
{
   int value = 0;              // Somewhere to accumulate the result

   value = number(str, index);    // Get the first number in the term

   // Loop as long as we have a good operator
   while( (str[index]=='*') || (str[index]=='/') || (str[index]=='%') )
   {

      if(str[index]=='*')                  // If it's multiply,
         value *= number(str, ++index);      // multiply by next number

      if(str[index]=='/')                  // If it's divide,
         value /= number(str, ++index);      // divide by next number

      if(str[index]=='%')
         value = value % number(str, ++index);
   }
   return value;             // We've finished, so return what we've got
}


// Function to recognize an expression in parentheses
// or a number in a string
int number(string &str, int& index)
{
   int value = 0;              // Store the resulting value

   if(str[index] == '(')             // Start of parentheses
   {
      string psubstr;               // Pointer for substring
      psubstr = extract(str, ++index); // Extract substring in brackets
      //cout << "psubstr: " << psubstr << endl;
      value = expr(psubstr);           // Get the value of the substring
      return value;                    // Return substring value
   }

    //cout << "number value: " << value << endl;
   while(isdigit(str[index]))     // Loop accumulating leading digits
      value=10*value + ((str[index++]) - 48);

                                     // Not a digit when we get to here
   return value;                     // On loop exit we are done
}


// Function to extract a substring between parentheses
// (requires string)
string extract(string &str, int& index)
{
   string pstr;             // Pointer to new string for return
   int numL = 0;             // Count of left parentheses found
   int bufindex = index;     // Save starting value for index

   do
   {
      switch(str[index])
      {
         case ')':
            if(numL==0)
            {
               ++index;
               pstr = str.substr(bufindex, index - bufindex-1);
               //str.erase(bufindex - 1, index - bufindex + 1);
               //cout << "extract pstr: " << pstr << endl;
               //cout << "str: " << str << endl;

               return pstr;          // Return substring in new memory
            }
            else
               numL--;           // Reduce count of '(' to be matched
            break;

         case '(':
            numL++;              // Increase count of '(' to be matched
            break;
      }
   } while(index++ < str.length());// Loop - don't overrun end of string

   cout << "Ran off the end of the expression, must be bad input."
        << endl;
   exit(1);
   return pstr;
}